Co-planar and microstrip waveguide bandpass filter

ABSTRACT

A bandpass filter for the transmission of signals within a predetermined frequency bandwidth having a center frequency, which provides for substantial attenuation of the harmonic components of the center frequency of the filter. The bandpass filter includes at least one resonator comprising a strip conductor and a ground conductor formed on the surface of a dielectric substrate. The strip conductor is capacitively coupled to the ground conductor so as to substantially transmit the harmonic components of the center frequency of the filter to ground.

BACKGROUND OF THE INVENTION

The present invention is directed to a bandpass filter formed on adielectric substrate which reduces the transmission of unwanted noise.

Bandpass filters are often utilized in various types of communicationequipment, for example, television receivers and cellular phones. Suchfilters discriminate between signals by passing signals in a desired,predetermined frequency band (i.e. pass the signal from the input to theoutput of the filter unattenuated), while preventing the transmission ofsignals outside the predetermined frequency band. In the event signalsoutside the desired frequency band are transmitted by the filter (i.e.the signals pass through the filter unattenuated), system performance isdegraded.

FIG. 16 is a schematic plan view illustrating a prior art bandpassfilter. Referring to FIG. 16, the prior art filter contains a substrate1 formed from a dielectric substance. The dielectric substrate 1functions as a support for three coplanar waveguide resonators 2-4,hereinafter referred to as a resonator, an input electrode 5, and anoutput electrode 6. Each resonator 2-4 comprises a strip conductor2a-4a. The strip conductor 2a of resonator 2 is coupled to the inputelectrode 5 through capacitor 7. The strip conductor 4a of resonator 4is coupled to the output electrode 6 through capacitor 8. Finally, stripconductor 3a of resonator 3 is coupled to the strip conductor 2a ofresonator 2 and strip conductor 4a of resonator 4 through capacitors 9and 10, respectively.

As is well known in the art, the frequency response of a filter definesthe level of attenuation of an input signal over the entire frequencyspectrum. The frequency response of the prior art bandpass filter ofFIG. 16 is illustrated in FIG. 15. Specifically, FIG. 15 depicts thelevel of attenuation of a signal by the prior art filter which has apredetermined center frequency equal to fo. The line designated by theletter "A" represents the minimum level of attenuation acceptable forsignals having a frequency outside a predefined frequency bandwidth ofthe filter. Frequency values 3 fo, 5 fo and 7 fo represent harmoniccomponents of the center frequency, fo, of the filter.

As is apparent from FIG. 15, signals having a frequency equal to theharmonic components 3 fo, 5 fo and 7 fo are not adequately attenuated bythe prior art filter. Although not shown, higher odd multiple harmoniccomponents of the center frequency of the filter (i.e. 9 fo, 11 fo,etc.) also traverse the filter without adequate attenuation. Thus, theprior art filter does not sufficiently attenuate all signals having afrequency outside the filter bandwidth centered about fo, which resultsin the generation of noise and the degradation of the performance of asystem utilizing such filters.

For example, typically a transmitter broadcasts signals of a singlepredetermined frequency, fo. This selective transmission is accomplishedby filters, which attenuate signals having a frequency other than fo.However, if the prior art filter is used in such a transmitter, thetransmitter will also output signals having a frequency equal to the oddharmonic components of fo, thereby transmitting unwanted signals (i.e.noise). Similarly, if the prior art filter is used in a receiver, itwill allow for the receipt of unwanted signals having a frequency equalto the harmonic components of the center frequency of the filter (i.e.noise).

Accordingly, there exists a present need for a bandpass filtercomprising coplanar waveguide resonators formed on a dielectricsubstrate which exhibits a frequency response that adequately attenuatesthe harmonic components of the center frequency of the filter so as toprevent the generation of noise.

SUMMARY OF THE INVENTION

According to this invention, a bandpass filter for the transmission ofsignals within a predetermined frequency bandwidth having a centerfrequency is provided, wherein the harmonic components of the centerfrequency of the filter are substantially attenuated by the filter so asto prevent the transmission of the harmonic components. The bandpassfilter of the present invention comprises at least one coplanarwaveguide resonator which is formed on a first surface of a dielectricsubstrate. The coplanar waveguide resonator comprises a strip conductorpositioned between a first ground conductor so as to form a transmissionline. The bandpass filter further comprises a second ground conductorformed on the first surface of the dielectric substrate. The secondground conductor is capacitively coupled to the strip conductor of thecoplanar waveguide resonator, wherein the harmonic components of thecenter frequency of the filter are substantially transmitted to thesecond ground conductor so as to substantially prevent the transmissionof the harmonic components by the filter.

As pointed out in greater detail below, the bandpass filter of thepresent invention provides important advantages over the bandpassfilters of the prior art. Specifically, the transmission level of theharmonic components of the center frequency of the bandpass filter issubstantially attenuated. Therefore, a system utilizing such a filter,for example, a transmitter or receiver, will exhibit a significantreduction in the noise generated by the transmission of harmoniccomponents associated with the center frequency of the filter. Thisreduction in the generation of noise allows for an increase in systemperformance. In addition, the elements of the bandpass filter (i.e.resonators, electrodes, ground planes) can be formed on a single plane,which significantly simplifies the production process. Another advantageis that the design of the present invention allows for a reduction inthe length of the resonator, which thereby allows for a reduction in theoverall size of the device. The bandpass filter of the present inventioncan be utilized within the frequency range of approximately 400 Mhz to 3Ghz.

The invention itself, together with further objects and attendantadvantages, will best be understood by reference to the followingdetailed description, taken in conjunction with the accompanyingdrawings.

BRIEF SUMMARY OF THE DRAWINGS

FIG. 1 is a schematic plan view of the structure of a first embodimentof the bandpass filter of the present invention.

FIG. 2 is a schematic plan view of the structure of a second embodimentof the bandpass filter of the present invention.

FIG. 3 is a schematic plan view of the structure of a third embodimentof the bandpass filter of the present invention.

FIG. 4 is a schematic plan view of the structure of a fourth embodimentof the bandpass filter of the present invention.

FIG. 5 is a graph illustrating the frequency response of the bandpassfilter of the first embodiment of the present invention.

FIG. 6 is a schematic front view of the structure of a fifth embodimentof the bandpass filter of the present invention.

FIG. 7 is a cross sectional view of the fifth embodiment of the presentinvention taken along the line X--Y of FIG. 6.

FIG. 8 is a schematic plan view of the structure of a sixth embodimentof the bandpass filter of the present invention.

FIG. 9 is a graph illustrating the frequency response of the bandpassfilter of the sixth embodiment of the present invention.

FIG. 10 is a schematic plan view of the structure of a seventhembodiment of the bandpass filter of the present invention.

FIG. 11 is a cross sectional view of the seventh embodiment of thepresent invention taken along the line X--Y of FIG. 10.

FIG. 12 is an enlarged front view of the primary part the seventhembodiment of the present invention illustrated in FIG. 11.

FIGS. 13 and 14 illustrate physical dimensions of the filter componentsof an exemplary filter formed according to the first embodiment of thepresent invention.

FIG. 15 is a graph illustrating the frequency response of the a priorart bandpass filter.

FIG. 16 is a schematic plan view of the structure of a prior artbandpass filter.

FIG. 17 is a schematic plan view of a first embodiment of a bandpassfilter of the present invention having one ground conductor.

DETAILED DESCRIPTION OF THE INVENTION

Turning now to the drawings, FIG. 1 is a schematic plan view showing theprimary part of the first embodiment of the bandpass filter of thepresent invention, hereinafter referred to as "the filter." As shown inFIG. 1, the first embodiment of the filter comprises a substrate 11formed from a dielectric substance. The filter further comprises threecoplanar waveguide resonators 12, 13 and 14, an input electrode 15, anoutput electrode 16, and a second ground conductor 17, all of which aredisposed on a first surface of the dielectric substrate as shown inFIG. 1. Each coplanar waveguide resonator 12, 13 and 14 comprises astrip conductor 12a, 13a, and 14a, having a flat bar shape, which ispositioned between first ground conductors 117a, 117b, 117c, 117d.Specifically, the strip conductor 12a associated with the first coplanarwaveguide resonator 12 is positioned between first ground conductors117a and 117b. The strip conductor 13a associated with the secondcoplanar waveguide resonator 13 is positioned between first groundconductors 117b and 117 c. Finally, the strip conductor 14a associatedwith the third coplanar waveguide resonator 14 is positioned betweenfirst ground conductors 117c and 117d. As shown, one end of each stripconductor 12a, 13a, 14a is integral with the first ground conductor soas to form a one end grounded type resonator. The strip conductor mayalso be referred to as a center conductor.

Preferably, all the conductors such as the coplanar waveguide resonator,the input and output electrodes, and the first and second groundconductors 17 are formed from copper foil.

Furthermore, the strip conductor 12a of the first coplanar waveguideresonator 12 is electrically coupled to the input electrode 15 by meansof a first capacitor 18. In a similar manner, the strip conductor 14a ofthe third coplanar waveguide resonator 14 is electrically coupled to theoutput electrode 16 by means of a second capacitor 19. The stripconductor 13a of the second coplanar waveguide resonator 13 iselectrically coupled to strip conductors 12a and 14a by a thirdcapacitor 20 and fourth capacitor 21, respectively. Each strip conductor12a, 13a and 14a is electrically coupled to the second ground conductor17 through a capacitor 22, 23, 24. Specifically, as shown in FIG. 1,strip conductor 12a is coupled to the second ground conductor 17 viacapacitor 22. Strip conductor 13a is coupled to the second groundconductor 17 via capacitor 23. And strip conductor 14a is coupled to thesecond ground conductor 17 via capacitor 24. The values of theaforementioned capacitors vary in accordance with the desired filterperformance.

A portion of the first ground conductor 117b situated between stripconductor 12a and strip conductor 13a extends towards the second groundconductor 17 so as to form an extending ground conductor 26. Similarly,a portion of the first ground conductor 117c situated between stripconductor 13a and strip conductor 14a extends towards the second groundconductor 17 so as to form an extending ground conductor 27. Theextending ground conductors 26, 27 function to prevent the transmissionof noise via an inherent capacitive component which functions to coupleadjacent resonators to one another.

In a variation of the present embodiment, the attenuation of the noiseattributable to this inherent capacitive component can be increased bylengthing the extending ground conductors 26, 27 so that each extendingground conductor is connected to the second ground conductor 17. Foreven further suppression of this noise component, the width of theextending ground conductors 26, 27 can be increased.

The length of the strip conductors 12a, 13a, 14a is selected so as tonegate a shift of the center frequency of the filter caused by thepresence of capacitors 22, 23 and 24 which are coupled between the stripconductors 12a, 13a and 14a and the second ground conductor 17, asdescribed above.

More specifically, capacitors 22, 23 and 24 provide an increase in thecapacitive component of the filter thereby causing a decrease (i.e.shift) in the center frequency, fo, of the filter. This shift in thecenter frequency of the filter is corrected by decreasing the inductivecomponent of the filter by trimming the length of the strip conductors12a, 13a and 14a. Accordingly, the center frequency of the filter ismaintained at the desired frequency.

Alternatively, if it is desirable to reduce the length of the coplanarwaveguide resonators 12, 13, 14, this can be accomplished by increasingthe value of capacitors 22, 23 and 24. Specifically, an increase in thevalue of capacitors 22, 23 and 24 results in an increase in thecapacitive component of the filter thereby decreasing the centerfrequency of the filter. The center frequency is restored to the desiredvalue by a reduction in the inductive component of the filter, which iscaused by reducing the length of the coplanar waveguide resonators 12,13, 14. Furthermore, if there are any fluctuations in the length of theresonators 12, 13, 14, such fluctuations can be compensated for byvarying the value of the associated capacitor 22, 23, 24. In otherwords, the center (i.e. resonant) frequency of the filter can beadjusted by varying the capacitive component of the filter.

The frequency response of the filter of the first embodiment is shown inFIG. 5, wherein the line designated by the letter "A" represents theminimum level of attenuation acceptable for signals having a frequencyoutside a predefined frequency bandwidth of the filter, and the letter"B" designates a harmonic component of the center frequency of thefilter. As is apparent, the lower order harmonic components (i.e 3 fo, 5fo, 7 fo) are extremely attenuated as compared to the prior art filterdescribed above. More specifically, the frequency response of the filterof the present invention is such that the lower order harmonicscomponents are more attenuated than the higher order harmoniccomponents, for example harmonic component B in FIG. 5. The increasedattenuation of the harmonic components is caused by capacitors 22, 23and 24 which function to transfer the harmonic components to the secondground conductor 17 thereby preventing the transmission of the harmoniccomponents. As the frequency of an input signal increases, the impedancecomponent associated with capacitors 22, 23 and 24 decreases therebyenhancing the transmission of higher frequency signals, which includeharmonic components, to ground.

The magnitude of the attenuation of the harmonic components of thecenter frequency of the filter varies in accordance with numerousfactors such as the value of the capacitors 22, 23, 24 coupling thestrip conductors 12a, 13a and 14a to the second ground conductor 17.Specifically, as the value of these capacitors 22, 23, 24 increase, theimpedance component associated with these capacitors decrease. As aresult of the decreased impedance, the harmonic components of the centerfrequency of the filter are more readily transmitted to ground.

Accordingly, a transmitter or receiver utilizing this filter willexhibit a significant reduction in the noise generated by thetransmission of harmonic components associated with the center frequencyof the filter. Also, as set forth above, the extending ground conductors26 and 27 prevent capacitive coupling between the first and secondcoplanar waveguide resonators 12, 13 and the second and third coplanarwaveguide resonators 13, 14 thereby contributing to the suppression ofnoise.

In a variation of the first embodiment of the present invention, thebandpass filter can comprise a single coplanar waveguide resonator. Forexample, the first and second coplanar waveguide resonators 12, 13 ofthe first embodiment can be eliminated. In addition, the capacitorsassociated with the first and second coplanar waveguide resonators 12and 13, namely the capacitors coupling the coplanar waveguide resonatorto the adjacent coplanar waveguide resonator and the second groundconductor, can also be eliminated. The frequency response of thismodified filter is equivalent to the frequency response of the filter ofthe first embodiment of the present invention.

Alternatively, the filter of the first embodiment can also comprise twoor more than three of the coplanar waveguide resonators described in thefirst embodiment of the present invention. In addition, a filtermodified as such would comprise the corresponding number of capacitorsnecessary for connecting the respective strip conductor of each coplanarwaveguide resonator to the adjacent strip conductor, and a capacitorconnecting each strip conductor to the second ground conductor asdescribed in the first embodiment.

Other variations of the first embodiment of the present invention arealso possible. For example, the strip conductors can be electricallycoupled to the first ground conductor, as opposed to the second groundconductor.

In one variation of the first embodiment, when the distance between theinput electrode 15 and the strip conductor 12a is sufficiently small,the first capacitor 18 can be replaced by the capacitance existingbetween the input electrode 15 and the strip conductor 12a. This"distance" varies in accordance with the desired characteristics of thefilter. Similarly, when the distance between the output electrode 16 andthe strip conductor 14a is sufficiently small, as defined by the filtercharacteristics, the second capacitor 19 can be replaced by thecapacitance existing between the output electrode 16 and the stripconductor 14a.

In another variation, as with the first and second capacitor 18, 19,when the distance between strip conductor 12a and strip conductor 13aand between strip conductor 13a and strip conductor 14a is sufficientlysmall, as defined by the filter characteristics, the third capacitor 20and the fourth capacitor 21 can be replaced by the capacitance existingbetween the respective strip conductors. Also, capacitors 22, 23 and 24can be replaced by the capacitance existing between the second groundconductor 17 and the respective strip conductor 12a, 13a, 14a, when thedistance between the second ground conductor 17 and the respective stripconductor 12a, 13a, 14a is sufficiently small, as defined by the filtercharacteristics.

In another variation, at least one of either the input electrode, theoutput electrode or the coplanar waveguide resonator can be disposed onthe opposite surface of the substrate.

In yet another variation, it is possible to eliminate the second groundconductor if each strip conductor forming the filter is electricallycoupled to the first ground conductor. As shown in FIG. 17. elementsequivalent to those described above and shown in FIG. 1 are designatedby the same number. As is apparent, the structure shown in FIG. 17 isthe same as FIG. 1 with the exception that the structure in FIG. 17 doesnot have a second conductor and the capacitors 22, 23 and 24electrically couple the strip conductor 12a, 13a and 14a to portions117a, 117b, 117c and 117d of the ground conductor. More specifically,strip conductor 12a is coupled to the portion 117a and the portion 117bof the ground conductor through the two capacitors 22, respectively.Strip conductor 13a is coupled to the portion 117b and the portion 117cof the ground conductor through the two capacitors 23, respectively.Strip conductor 14a is coupled to the portion 117c and the portion 117dof the ground conductor through the two capacitors 24, respectively.

Finally, the electrodes and first and second ground conductors can bemade of silver or other conductive materials as well as copper foil.

A second embodiment of the bandpass filter of the present invention isillustrated in FIG. 2, wherein elements equivalent to those disclosed inthe first embodiment are designated by the same number. As is apparent,the structure of the second embodiment of the filter is the same as thestructure of the first embodiment. The difference between the twoembodiments is that in the second embodiment the capacitors 22, 23 and24, which electrically couple the strip conductors 12a, 13a and 14a tothe second ground conductor 17 have different capacitive values. Morespecifically, capacitor 23 which couples strip conductor 13a to thesecond ground conductor 17 has a larger capacitive value than capacitors22 and 24, and the strip conductor 13a of coplanar waveguide resonator13 exhibits a reduction in length. The length of the strip conductor 13ais reduced so as to decrease the inductive component of the filter suchthat the resonant frequency of filter, which is a function of both theinductive and capacitive components, remains at the desired value.

The frequency response of the filter of the second embodiment of thepresent invention is equivalent to the frequency response of the filterof the first embodiment.

A third embodiment of the bandpass filter of the present invention,which employs strip line type resonators, is illustrated in FIG. 3,wherein elements equivalent to those disclosed in the first embodimentare designated by the same number. As shown in FIG. 3, the bandpassfilter comprises three strip conductors 29, 30, 31, preferably having aflat bar shape, disposed at specified locations on one surface of asubstrate 28, as well as an input electrode 32, an output electrode 33and a second ground conductor 34 all of which are disposed at specifiedlocations on the same surface of the substrate 28 as the three stripconductors 29, 30, 31. The other (i.e. opposite) surface of thesubstrate 28 (not shown in FIG. 3) is completely covered with a firstground conductor (not shown). The combination of a strip conductor 29,30 or 31 separated from a first conductor (not shown) by a dielectricsubstrate 28 is a microstrip waveguide. The strip conductors 29, 30, 31and the second ground conductor 34 are connected at the respectiveproximal end to the first ground conductor disposed on the other side ofthe substrate 28, thereby forming resonators.

Capacitors 18-24 are arranged and operate in the same manner as thecorresponding capacitor disclosed in the first embodiment of the presentinvention. More specifically, the strip conductor 29 and the inputelectrode 32 are electrically coupled through a first capacitor 18.Strip conductor 31 and the output electrode 33 are electrically coupledthrough a second capacitor 19. Strip conductor 30 is electricallycoupled with strip conductors 29 and 31 by a third capacitor 20 and afourth capacitor 21, respectively.

Each strip conductor 29, 30, 31 is also electrically coupled to thesecond ground conductor 34 through a capacitor 22, 23, 24. Specifically,as shown in FIG. 3, strip conductor 29 is coupled to the second groundconductor 34 via capacitor 22. Strip conductor 30 is coupled to thesecond ground conductor 34 via capacitor 23. And strip conductor 31 iscoupled to the second ground conductor 34 via capacitor 24.

The frequency response of the filter of the third embodiment of thepresent invention is substantially equivalent to the frequency responseof the filter of the first embodiment.

Similar to the first embodiment of the present invention, the filter ofthe third embodiment is not limited to a structure comprising threestrip conductors. Variations of the third embodiment are possiblewherein the filter may comprise any number of strip conductors(including a single strip conductor), which are electrically coupled asdescribed above by the addition or elimination of capacitors asrequired.

In another variation of the third embodiment of the present invention,when the distance between the input electrode 32 and the strip conductor29 is sufficiently small, as defined by the filter characteristics, thefirst capacitor 18 can be replaced by the capacitance existing betweenthe input electrode 32 and the strip conductor 29. Similarly, when thedistance between the output electrode 33 and the strip conductor 31 issufficiently small, as defined by the filter characteristics, the secondcapacitor 19 can be replaced by the capacitance existing between theoutput electrode 33 and the strip conductor 31.

In yet another variation of the third embodiment, as with the first andsecond capacitor 18, 19, when the distance between strip conductor 29and strip conductor 30 and between strip conductor 30 and 31 issufficiently small, as defined by the filter characteristics, the thirdcapacitor 20 and the fourth capacitor 21 can be replaced by thecapacitance existing between the respective strip conductors. Also,capacitors 22, 23 and 24 can be replaced by the capacitance existingbetween the second ground conductor 34 and the respective stripconductor 29, 30, 31, when the distance between the ground conductor 34and the respective strip conductor 29, 30, 31 is sufficiently small, asdefined by the filter characteristics.

A fourth embodiment of the bandpass filter of the present invention isillustrated in FIG. 4, wherein elements equivalent to those disclosed inthe third embodiment are designated by the same number. As is apparent,the structure of the fourth embodiment of the filter is the same as thestructure of the third embodiment of the filter. The difference betweenthe two embodiments is that in the fourth embodiment the capacitors 22,23 and 24 which electrically couple the strip conductors 29, 30 and 31to the second ground conductor 34 have different capacitive values. Morespecifically, capacitor 23 which couples strip conductor 30 to thesecond ground conductor 34 has a larger capacitive value than capacitors22 and 24, and the strip conductor 30 exhibits a proportional reductionin length. The length of the strip conductor 30 is reduced so as todecrease the inductive component of the filter such that the resonantfrequency of filter, which is a function of both the inductive andcapacitive components, remains at the desired value.

The frequency response of the filter of the fourth embodiment of thepresent invention is substantially equivalent to the frequency responseof the filter of the third embodiment.

FIGS. 6 and 7 illustrate a fifth embodiment of the bandpass filter ofthe present invention, which comprises a plurality of substratesarranged in layers. Specifically, FIG. 6 is a front view of thestructure of the fifth embodiment of the filter, and FIG. 7 is across-sectional view of the structure of the fifth embodiment of thefilter taken along line X--Y of FIG. 6.

As shown in FIG. 6, the filter comprises three strip conductors 29a,30a, and 31a, an input electrode 32, and an output electrode 33, all ofwhich are disposed on the upper surface of a first substrate 28 formedfrom a dielectric substance. A ground conductor 35 is disposed on thelower surface of the first substrate 28.

The filter further comprises a second substrate 36 formed from adielectric substance which is disposed on the upper surface of the firstsubstrate 28. A ground conductor 34a is formed on the second substrate36 so as to extend across the distal ends of the strip conductors 29a,30a, 31a as denoted by the broken lines in FIG. 7. In other words, theground conductor 34a is separated from the respective distal ends of thestrip conductors 29a, 30a, 31a by the second substrate 36. Thecombination of a strip conductor 29a, 30a or 31a separated from a groundconductor 34a by the second dielectric substrate 36 is a microstripwaveguide resonator. An additional dielectric layer 37 and a groundconductor 38 are formed on the ground conductor 34a. Strip conductors29a-31a, ground conductors 35 and 38, and dielectric substrates 28 and37 form the resonator.

Capacitors 18-24 are arranged and operate in the same manner as thecorresponding capacitors disclosed in the first embodiment of thepresent invention. More specifically, the strip conductor 29a and theinput electrode 32 are electrically coupled through a first capacitor18. Strip conductor 31a and the output electrode 33 are electricallycoupled through a second capacitor 19. Strip conductor 30a iselectrically coupled with strip conductors 29a and 31a by a thirdcapacitor 20 and a fourth capacitor 21, respectively.

Each strip conductor 29a, 30a, 31a is also electrically coupled to thesecond ground conductor 34a through a capacitor 22, 23, 24.Specifically, as shown in FIG. 6, strip conductor 29a is coupled to thesecond ground conductor 34a via capacitor 22. Strip conductor 30a iscoupled to the second ground conductor 34a via capacitor 23. And stripconductor 31a is coupled to the second ground conductor 34a viacapacitor 24.

The frequency response of the filter of the fifth embodiment of thepresent invention is substantially equivalent to the frequency responseof the filter of the first embodiment.

Similar to the other embodiments of the present invention, the filter ofthe fifth embodiment is not limited to a structure comprising threestrip conductors. Variations of the fifth embodiment are possiblewherein the filter may comprise any number of strip conductors(including a single strip conductor), which are electrically coupled bythe addition or elimination of capacitors as described above.

Referring to FIG. 8, a sixth embodiment of the bandpass filter of thepresent invention is described. FIG. 8 is a plan view of a primary partof the structure of a 1/2 wavelength filter employing the filters of thepresent invention. The 1/2 wavelength filter comprises an inputelectrode 40, an output electrode 41 and three coplanar waveguideresonators 42, 43 and 44, all of which are disposed on a substrate 39formed from a dielectric substance.

Each coplanar waveguide resonator 42, 43, 44 comprises a strip conductor42a, 43a, 44a, having a flat bar shape, and ground conductors 45 and 46disposed opposite each other on both sides of the strip conductors 42a,43a, 44a. As shown in FIG. 8, the input electrode 40 is electricallycoupled to one end of the first strip conductor 42a through a capacitor47, and the other end of the first strip conductor 42a is electricallycoupled to a first end of the second strip conductor 43a through acapacitor 49. The other end of the second strip conductor 43a iselectrically coupled to a first end of the third strip conductor 44athrough a capacitor 50, and the other end of the third strip conductor44a is electrically coupled to the output electrode 41 through acapacitor 48.

Each strip conductor 42a, 43a, 44a are also electrically coupled to theground conductors 45 and 46 located adjacent both sides of the stripconductors 42a, 43a, 44a through a plurality of capacitors 51.Specifically, as shown in FIG. 8, each end of the strip conductors 42a,43a, 44a is connected to both ground conductor 45 and ground conductor46 through a separate capacitor 51. The present embodiment requirestwelve capacitors 51 to couple the three strip conductors 42a, 43a, 44ato the ground conductors 45, 46.

FIG. 9 is a diagram of the frequency response of the 1/2 wavelengthfilter shown in FIG. 8. Referring to FIG. 9, the line designated by theletter "A" represents the minimum level of attenuation acceptable forsignals having a frequency outside a predefined frequency bandwidth ofthe filter, and the letter "B" designates a harmonic component of thecenter frequency of the filter. As is apparent, the frequency responseof the filter is such that the attenuation of the harmonic components ofthe center frequency of the filter does not decrease until the higherorder harmonics (i.e., the harmonic component "B" in FIG. 9). However,the higher order harmonic component is substantially outside therequired bandwidth of the filter, and more importantly, does not exceedthe minimum level of attenuation designated by the letter "A".

The increased attenuation of the harmonic components is a function ofelectrically coupling each end of the strip conductors 42a, 43a, 44a toboth ground conductors 45 and 46. In addition, this coupling alsoprovides a filter which preserves the electrical phase stability of asignal.

It is of note that it is theoretical possible to obtain the samefrequency response as shown in FIG. 9 with the 1/2 wavelength filterdescribed in FIG. 8 modified so that each end of the strip conductors42a, 43a, 44a is electrically coupled to either ground conductor 45 orground conductor 46 through capacitor 51. However, in practice, such anembodiment of the 1/2 wavelength filter results in a loss of theelectrical phase balance thereby impairing an otherwise acceptablefrequency response.

Variations of the sixth embodiment of the present invention arepossible. For example, the 1/2 wavelength filter may comprise any numberof strip conductors (including a single strip conductor), wherein eachstrip conductor is electrically coupled to the adjacent strip conductorand to ground conductor 45 and ground conductor 46 by the addition orelimination of capacitors as described above with reference to FIG. 8.

Referring now to FIGS. 10-12, a seventh embodiment of the bandpassfilter of the present invention is described. FIG. 10 is a schematicplan view of a primary part of the structure of a 1/2 wavelength filteremploying strip conductors. FIG. 11 is a cross sectional view of the 1/2wavelength filter taken along line X--Y of FIG. 10. FIG. 12 is anenlarged view of a portion of FIG. 11.

As shown in FIGS. 10-12, the 1/2 wavelength filter comprises an inputelectrode 53, an output electrode 54 and three strip conductors 55, 56and 57, which are linearly disposed at specified intervals on the uppersurface of a substrate 52 formed from a dielectric substance. As shownin FIG. 12, the dielectric substrate 52 comprises a plurality of throughholes 52a which extend from the upper surface of the substrate 52 to thelower surface (i.e. opposite) of the substrate 52. The holes 52a arelocated on the substrate 52 such that when the strip conductors 55, 56,57 are disposed on the substrate 52, a single hole 52a underlies eachend of the strip conductors 55, 56, 57.

On the lower surface of the substrate 52, each hole 52a is covered by anelectrode 52b formed on the lower surface of the dielectric substrate52. The filter also comprises a plurality of ground conductors 58disposed on the lower surface of the substrate 52. The ground conductors58 are positioned opposite the input electrode 53, the output electrode54, and each of the three strip conductors 55, 56, 57. However, as shownin FIGS. 11-12, the ground conductor 58 does not extend the entirelength of the corresponding strip conductor 55, 56, 57 disposed on theupper surface of the substrate 52, because each ground conductor 58 isseparated from the electrode 52b which is formed directly beneath theends of each strip conductor 55, 56, 57.

As shown in FIG. 10, the input electrode 53 is electrically coupled to afirst end of strip conductor 55 through a first capacitor 59, and theopposite end of the strip conductor 55 is electrically coupled to afirst end of strip conductor 56 through capacitor 60. In a similarfashion, the opposite end of strip conductor 56 is electrically coupledto a first end of strip conductor 57 through capacitor 61, and theopposite end of strip conductor 57 is electrically coupled to the outputterminal 54 through capacitor 62. Also, as shown in FIG. 11, each groundconductor 58 positioned beneath a strip conductor 55, 56, 57 iselectrically coupled to the two adjacent lower electrodes 52b throughseparate capacitors 63.

The frequency response of the filter of the seventh embodiment of thepresent invention is substantially equivalent to the frequency responseof the filter of the sixth embodiment.

Similar to the sixth embodiment of the present invention, the filter ofthe seventh embodiment is not limited to a structure comprising threestrip conductors. Variations of the seventh embodiment are possiblewherein the filter may comprise any number of strip conductors(including a single strip conductor), which are electrically coupled bythe addition or elimination of capacitors and strip conductors asdescribed above.

In each of the aforementioned embodiments of the present invention, thephysical dimensions of the filter components vary in accordance withfactors including the dielectric constant of the substrate, the selectedcenter frequency of the filter, and the value of capacitor coupling theresonator to the second ground conductor.

FIGS. 13 and 14 illustrate the physical dimensions of the filtercomponents of an exemplary filter formed according to the firstembodiment of the present invention. This filter has a center frequencyof 1 Ghz. As is apparent, the dielectric substrate 11 measures 20 mm inwidth, 15 mm in length and 1.6 mm in thickness. Strip conductors 12a and14a measure 3.0 mm in length, while strip conductor 13a measures 4.5 mmin length. All three strip conductors 12a, 13a, 14a measure 0.8 mm inwidth and 0.018 mm in thickness. The distance between the first groundconductors, for example 117b and 117c, measures 1.75 mm. The distancebetween the strip conductors, for example 12a and 13a, measures 6.0 mm.The distance between the input electrode 15 and strip conductor 12ameasures 1.5 mm, as does the distance between the output electrode 16and strip conductor 14a. As previously stated, all of the aforementioneddimensions vary in accordance with the desired filter performance.

The embodiments described above provide significant advantages over theprior art bandpass filters. For example, because the strip conductorsforming the coplanar waveguide resonator are electrically coupled to theground conductor through capacitors, the resultant filter providesincreased attenuation of the harmonic components of the center frequencyof the filter. As a result, the bandpass filter of the present inventionsubstantially reduces the transmission of unwanted noise, therebyenhancing the performance of systems utilizing bandpass filters, such astelevision receivers and cellular phones.

Another advantage of the present invention is that the elements of thebandpass filter (i.e. resonators, electrodes, ground planes) can beformed on a single plane, which significantly simplifies the productionprocess. Yet another advantage is that the design of the presentinvention allows for a reduction in the length of the resonator, whichthereby allows for a reduction in the overall size of the device.

Of course, it should be understood that a wide range of changes andmodifications can be made to the preferred embodiments described above.It is therefore intended that the foregoing detailed description beregarded as illustrative rather than limiting, and that it be understoodthat it is the following claims, including all equivalents, which areintended to define the scope of this invention.

What is claimed is:
 1. A band pass filter for the transmission ofsignals within a predetermined frequency bandwidth having a centerfrequency, said filter comprising:a dielectric substrate, an inputelectrode formed on a surface of said dielectric substrate, an outputelectrode formed on a surface of said dielectric substrate, a firstground conductor formed on a surface of said dielectric substrate todefine an edge, said first ground conductor having at least one cavityextending inwardly from said edge of said first ground conductor, saidat least one cavity having an inner edge, a strip conductor extendingoutwardly from said inner edge of said at least one cavity, said stripconductor having an outer edge aligned in a straight line with said edgeof said first ground conductor, thereby forming at least one coplanarwaveguide resonator, a second ground conductor formed on a surface ofsaid dielectric substrate, a first capacitor electrically coupling saidinput electrode and said strip conductor forming said at least onecoplanar waveguide resonator, a second capacitor electrically couplingsaid output electrode and said strip conductor forming said at least onecoplanar waveguide resonator, and at least one further capacitorelectrically coupling said second ground conductor and said stripconductor forming said at least one coplanar wavequide resonator,wherein harmonic components of the center frequency of said filter aresubstantially transmitted to ground through said further capacitor toprevent transmission of said harmonic components by said filter.
 2. Abandpass filter according to claim 1, whereinsaid first ground conductorhas a plurality of cavities extending inwardly from said edge of saidfirst ground conductor, each of said cavities having an inner edge, astrip conductor extending outwardly from said inner edge of each cavity,having an outer edge aligned in a straight line with said edge of saidfirst conductor, thereby forming a plurality of coplanar waveguideresonators, said first capacitor electrically coupling said inputelectrode and a strip conductor forming one of said plurality ofwaveguide resonators, said second capacitor electrically coupling saidoutput electrode and another strip conductor forming another of saidplurality of waveguide resonators, said at least one further capacitorincludes a further capacitor for each further waveguide resonator, withsaid further capacitor electrically coupling said second groundconductor to a further strip conductor forming a further waveguideresonator of said plurality of waveguide resonators, wherein each saidfurther waveguide resonator is electrically coupled to said secondground conductor by a further capacitor, and further comprising,additional capacitors, wherein an additional capacitor of saidadditional capacitors electrically couples said strip conductor formingsaid one of said plurality of waveguide resonators to a further stripconductor forming a further waveguide resonator, and another additionalcapacitor of said additional capacitors electrically couples saidanother strip conductor forming said another of said plurality ofwaveguide resonators to a further strip conductor forming a furtherwaveguide resonator, with any further strip conductor being electricallycoupled to another strip conductor by a further additional capacitor ofsaid additional capacitors, whereby each strip conductor is electricallycoupled to another strip conductor by an additional capacitor.
 3. Abandpass filter according to claim 2, whereinthe capacitive value ofeach said further capacitor electrically coupling said second groundconductor and a strip conductor depends on the desired center frequencyof the filter, wherein the center frequency of said bandpass filter canbe adjusted by varying the capacitive value of each said furthercapacitor.
 4. A bandpass filter according to claim 2, whereintheinductive value of each coplanar waveguide resonator depends on thelength of each said strip conductor and the desired center frequency ofthe filter, wherein the center frequency of said bandpass filter can beadjusted by Varying the length of each said strip conductor.
 5. Abandpass filter according to claim 2, further comprising,an extendingground conductor for each portion of said first ground conductor formedbetween a pair of said strip conductors electrically coupling said eachportion to said second conductor for preventing the transmission ofnoise and preventing a coupling capacitance between said a pair of saidstrip conductors.
 6. A bandpass filter according to claim 5, whereinthelength and width of each extending ground conductor depends on thedesired amount of noise suppression.
 7. A band pass filter for thetransmission of signals within a predetermined frequency bandwidthhaving a center frequency, said filter comprising:a dielectricsubstrate, an input electrode formed on a surface of said dielectricsubstrate, an output electrode formed on a surface of said dielectric,substrate a ground conductor formed on a surface of said dielectricsubstrate to define an edge, said ground conductor having at least onecavity extending inwardly from said edge of said ground conductor, saidat least one cavity having an inner edge, a strip conductor extendingoutwardly from said inner edge of said at least one cavity, said stripconductor having an outer edge aligned in a straight line with said edgeof said ground conductor, thereby forming at least one coplanarwaveguide resonator, a first capacitor electrically coupling said inputelectrode and said strip conductor forming said at least one coplanarwaveguide resonator, a second capacitor electrically coupling saidoutput electrode and said strip conductor forming said at least onecoplanar wavequide resonator, and at least one further capacitorelectrically coupling said ground conductor and said strip conductorforming said at least one coplanar wavequide resonator, wherein harmoniccomponents of the center frequency of said filter are substantiallytransmitted to ground through said further capacitor to preventtransmission of said harmonic components by said filter.
 8. A bandpassfilter according to claim 7, whereinsaid ground conductor has aplurality of cavities extending inwardly from said edge of said groundconductor, each of said cavities having an inner edge, a strip conductorextending outwardly from said inner edge of each cavity, having an outeredge aligned in a straight line with said edge of said conductor,thereby forming a plurality of coplanar waveguide resonators, said firstcapacitor electrically coupling said input electrode and a stripconductor forming one of said plurality of waveguide resonators, saidsecond capacitor electrically coupling said output electrode and anotherstrip conductor forming another of said waveguide resonators, said atleast one further capacitor includes further capacitor for each furtherwaveguide resonator, with a said further capacitor electrically couplingsaid ground conductor to a further strip conductor forming a furtherwaveguide resonator of said plurality of waveguide resonators, whereineach said further waveguide resonator is electrically coupled to saidground conductor by a further capacitor, and further comprising,additional capacitors wherein an additional capacitor of said additionalcapacitors electrically couples said strip conductor forming said one ofsaid plurality of waveguide resonators to a further strip conductorforming a further waveguide resonator, and another additional capacitorof said additional capacitors electrically couples said another stripconductor forming said another of said plurality of waveguide resonatorsto a further strip conductor forming a further waveguide resonator, withany further strip conductor being electrically coupled to another stripconductor by a further additional capacitor of said additionalcapacitors, whereby each strip conductor is electrically coupled toanother strip conductor by an additional capacitor.
 9. A bandpass filteraccording to claim 8 whereinthe capacitive value of each said furthercapacitor electrically coupling said ground conductor and a stripconductor depends on the desired center frequency of the filter, whereinthe center frequency of said bandpass filter can be adjusted by varyingthe capacitive value of each said further capacitor.
 10. A bandpassfilter according to claim 8, whereinthe inductive value of each coplanarwaveguide resonator depends on the length of each said strip conductorand the desired center frequency of the filter, wherein the centerfrequency of said bandpass filter can be adjusted by varying the lengthof each said strip conductor.
 11. A band pass filter for thetransmission of signals within a predetermined frequency bandwidthhaving a center frequency, said filter comprising:a first dielectricsubstrate with a first surface defining an edge, an input electrodeformed on said first surface of said first dielectric substrate, anoutput electrode formed on said first surface of said first dielectricsubstrate, a first ground conductor formed on a second surface of saidfirst dielectric substrate, at least one microstrip waveguide resonatorhaving a strip conductor shaped as a flat bar, having one end terminatedat said edge of said first surface of said first dielectric substrateand connected to said first ground conductor, a second dielectricsubstrate disposed over said first surface of said first dielectricsubstrate, said second dielectric substrate having a first and a secondsurface, said first surface of said first dielectric substrate facingsaid first surface of said second dielectric substrate, a second groundconductor shaped as a bar formed on said second surface of said seconddielectric substrate and positioned above the other end of the stripconductor, a first capacitor electrically coupling said input electrodeand the other end of said strip conductor forming said at least onemicrostrip waveguide resonator, a second capacitor electrically couplingsaid output electrode and the other end of said strip conductor formingsaid at least one microstrip waveguide resonator, and at least onefurther capacitor for adjusting the center frequency of said filter,said at least one capacitor electrically coupling said second groundconductor and the other end of said strip conductor forming said atleast one microstrip waveguide resonator, wherein harmonic components ofthe center frequency of said filter are substantially transmitted toground through said further capacitor to prevent transmission of saidharmonic components by said filter.
 12. A bandpass filter according toclaim 11, wherein,said at least one microstrip waveguide resonatorincludes a plurality of said microstrip waveguide resonators, eachwaveguide having a strip conductor shaped as a flat bar, each stripconductor having one end terminating at said edge of said first surfaceof said first dielectric and connected to said first ground conductor,said first capacitor electrically coupling said input electrode and theother end of a strip conductor forming one of a plurality of saidwaveguide resonators, said second capacitor electrically coupling saidoutput electrode and the other end of another strip conductor forminganother of a plurality of said waveguide resonators, said at least onefurther capacitor includes a further capacitor for each furtherwaveguide resonator, with said further capacitor electrically couplingsaid second qround conductor to said other end of a further stripconductor forming a further waveguide resonator of said plurality ofwaveguide resonators, wherein each said further waveguide resonator iselectrically coupled to said second ground conductor by a furthercapacitor, and further comprising, additional capacitors, wherein anadditional capacitor of said additional capacitors electrically couplessaid other end of said a strip conductor forming said one of saidplurality of waveguide resonators to said other end of a further stripconductor forming a further waveguide resonator, and another additionalcapacitor of said additional capacitors electrically couples said otherend of said another strip conductor forming said another of saidplurality of waveguide resonators to said other end of a further stripconductor forming a further waveguide resonator, with the other end ofany further strip conductor being electrically coupled to the other endof another strip conductor by a further additional capacitor of saidadditional capacitors, whereby each strip conductor is electricallycoupled to another strip conductor by an additional capacitor.
 13. Abandpass filter according to claim 12, whereinthe capacitive value ofeach said further capacitor electrically coupling said second groundconductor and a strip conductor depends on the desired center frequencyof the filter, wherein the center frequency of said bandpass filter canbe adjusted by varying the capacitive value of each said furthercapacitor.
 14. A bandpass filter according to claim 12, whereintheinductive value of each microstrip waveguide resonator depends on thelength of each said strip conductor and the desired center frequency ofthe filter, wherein the center frequency of said bandpass filter can beadjusted by varying the length of each said strip conductor.
 15. A bandpass filter for the transmission of signals within a predeterminedfrequency bandwidth having a center frequency, said filter comprising:adielectric substrate, an input electrode formed on a first surface ofsaid dielectric substrate, an output electrode formed on said firstsurface of said dielectric substrate, at least one coplanar waveguideresonator having a strip conductor disposed on said first surface ofsaid dielectric substrate in a straight line between said inputelectrode and said output electrode, said strip conductor having an endand an other end a first ground conductor formed on said first surfaceof said dielectric, a second ground conductor formed on said firstsurface of said dielectric, said first and second ground conductorsdisposed opposite each other on both sides of said straight lineincluding a strip conductor, said input electrode and said outputelectrode, a first capacitor electrically coupling said input electrodeto said strip conductor forming said at least one coplanar waveguideresonator at an end adjacent said input electrode, a second capacitorelectrically coupling said output electrode to said strip conductorforming said at least one coplanar waveguide resonator at an other endadjacent said output electrode, and four capacitors for each stripconductor, a first of said four capacitors electrically coupling an endof a strip conductor to said first ground conductor, a second of saidfour capacitors electrically coupling an end of a strip conductor tosaid second ground conductor, a third of said four capacitorselectrically coupling an other end of a strip conductor to said firstground conductor, and a fourth of said four capacitors electricallycoupling an other end of a strip conductor to said second groundconductor, wherein harmonic components of the center frequency of saidfilter are substantially transmitted to ground through said furthercapacitors to prevent transmission of said harmonic components by saidfilter.
 16. A bandpass filter according to claim 15, wherein,said atleast one coplanar waveguide resonator includes a plurality of saidcoplanar waveguide resonators, each waveguide resonator having a stripconductor disposed on said first surface of said dielectric substrate ina straight line between said input electrode and said output electrode,and further comprising, additional capacitors, wherein an additionalcapacitor of said additional capacitors electrically couples an end ofsaid strip conductor forming said one of said plurality of waveguideresonators to an other end of a further strip conductor forming afurther waveguide resonator, and another additional capacitor of saidadditional capacitors electrically couples an end of said another stripconductor forming said another of said plurality of waveguide resonatorsto an other end of a further strip conductor forming a further waveguideresonator, with an end of any further strip conductor being electricallycoupled to one other end of another strip conductor by a furtheradditional capacitor of said additional capacitors, whereby each stripconductor is electrically coupled to another strip conductor by anadditional capacitor.
 17. A bandpass filter according to claim 16,whereinthe capacitive value of each capacitor of said four capacitorselectrically coupling the ground conductors and a strip conductordepends on the desired center frequency of the filter.
 18. A bandpassfilter according to claim 16, whereinthe inductive value of eachcoplanar waveguide resonator depends on the length of each said stripconductor and the desired center frequency of the filter, said lengthdiffering for at least two strip conductors.
 19. A band pass filter forthe transmission of signals within a predetermined frequency bandwidthhaving a center frequency, said filter comprising:a dielectric substratehaving a plurality of holes extending from a first surface to a secondopposite surface of said dielectric substrate, an input electrode formedon said first surface of said dielectric substrate, an output electrodeformed on said first surface of said dielectric substrate, at least onewaveguide resonator having a strip conductor, disposed on said firstsurface of said dielectric substrate in a straight line between saidinput electrode and said output electrode said strip conductor having anend and an other end with each end of said strip conductor disposedadjacent a hole in said dielectric substrate, a ground conductor formedon a second surface of said dielectric, a first capacitor electricallycoupling said input electrode to said strip conductor forming said atleast one waveguide resonator at an end adjacent said input electrode, asecond capacitor electrically coupling said output electrode to saidstrip conductor forming said at least one waveguide resonator at another end adjacent said output electrode, and two capacitors for eachstrip conductor, a first of said two capacitors electrically coupling anend of a strip conductor through a hole to said ground conductor, and asecond of said two capacitors electrically coupling an other end of astrip conductor through a hole to said ground conductor, whereinharmonic components of the center frequency of said filter aresubstantially transmitted to ground through said further capacitor toprevent transmission of said harmonic components by said filter.
 20. Abandpass filter according to claim 19, wherein,a said at least onewaveguide includes a plurality of said waveguides, each waveguide havinga strip conductor disposed on said first surface of said dielectricsubstrate in a straight line between said input electrode and saidoutput electrode, and further comprising, additional capacitors,whereinan additional capacitor of said additional capacitors electricallycouples an end of said strip conductor forming said one of saidplurality of waveguide resonators to an other end of a further stripconductor forming a further waveguide resonator, and another additionalcapacitor of said additional capacitors electrically couples an end ofsaid another strip conductor forming said another of said plurality ofwaveguide resonators to an other end of a further strip conductorforming a further waveguide resonator, with an end of any further stripconductor being electrically coupled to one other end of another stripconductor by a further additional capacitor of said additionalcapacitors, whereby each strip conductor is electrically coupled toanother strip conductor by an additional capacitor.
 21. A bandpassfilter according to claim 20, whereinthe capacitive value of eachcapacitor of said two capacitors electrically coupling said groundconductor and a strip conductor depends on the desired center frequencyof the filter.
 22. A bandpass filter according to claim 20, whereintheinductive value of each waveguide resonator depends on the length ofeach said strip conductor and the desired center frequency of thefilter, said length differing for at least two strip conductors.